The present disclosure relates to nitride semiconductor devices and more particularly to nitride semiconductor devices and applicable as power transistors in power supply circuits etc.
Nitride semiconductor represented by gallium nitride (GaN) draws attention as a material of a high-frequency semiconductor device or a high-output semiconductor device. The use of a silicon (Si) substrate for fabrication of nitride semiconductor devices is considered. A large-size Si substrate can be easily provided. If a Si substrate is used as a substrate for growing nitride semiconductor, costs of manufacturing a nitride semiconductor device can be largely reduced.
Operations of a nitride semiconductor device formed on a Si substrate are affected by the potential of the Si substrate. Thus, in order to stabilize the potential of the Si substrate, an electrode formed on the back surface of the Si substrate is connected to a source electrode or a drain electrode via a through electrode which penetrates a nitride semiconductor layer and the Si substrate. When the back surface electrode is electrically connected to the source electrode or the drain electrode, a high voltage is applied between the back surface electrode and the source electrode or the drain electrode. It is thus necessary to increase the breakdown voltage of the semiconductor device in the longitudinal direction. The breakdown voltage of the semiconductor device on the Si substrate in the longitudinal direction depends on the breakdown voltage of the nitride semiconductor layer grown on the Si substrate, and the breakdown voltage of the Si substrate. In order to increase the breakdown voltage of the nitride semiconductor layer, the thickness of the nitride semiconductor layer needs to be increased. However, a thickness of a nitride semiconductor layer which can be formed on a Si substrate is limited, since lattice constants and thermal expansion coefficients of Si and nitride semiconductor are significantly different.
Therefore, improving the breakdown voltage of a semiconductor device in the longitudinal direction by increasing the breakdown voltage of a Si substrate is considered. (See, for example, Japanese Patent Publication No. 2005-217049).